The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years. In these applications, the coating material is discharged in atomized form, and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged atomized particles. In the past, coating materials of the solvent-based variety, such as varnishes, lacquers, enamels, and the like, were the primary materials employed in electrostatic coating applications. The problem with such coating materials is that they create an atmosphere which is both explosive and toxic. The explosive nature of the environment presents a safety hazard should a spark inadvertently be generated, such as by accidentally grounding the nozzle of the spray gun, which can ignite the solvent in the atmosphere causing an explosion. The toxic nature of the workplace atmosphere created by solvent coating materials can be a health hazard should an employee inhale solvent vapors.
As a result of the problems with solvent-based coatings, the recent trend has been to switch to water-based coatings which reduce the problems of explosiveness and toxicity. Unfortunately, the switch from electrostatically spraying solvent-based coatings to those of the water-based type has sharply increased the risk of electrical shock, which risk was relatively minor with solvent-based coatings. The risk of electrical shock is occasioned in the use of water-based coatings due to their extreme electrical conductivity, with resistivities of such water-based coatings often falling within the range of 100 to 10,000 ohm centimeters. This is in contrast to resistivities of 200,000 to 100,000,000 ohm centimeters for moderately electrically conductive coatings such as metallic paint, and resistivities exceeding 100,000,000 ohm centimeters for solvent-based lacquers, varnishes, enamels and the like.
The relative resistivity of the coating material is critical to the potential electrical shock which may arise during an electrostatic coating operation. With coating materials which are either not electrically conductive or only moderately electrically conductive, the column of coating material which extends from the charging electrode at the tip of the coating dispenser through the hose leading back to the supply tank has sufficient electrical resistance to prevent any significant electrostatic charging of the material in the supply tank or the tank itself. However, when coating material is highly electrically conductive, as are water-based coatings, the resistance of the coating column in the supply hose is very low. As a result, a high voltage charging electrode located in the vicinity of the nozzle of the coating dispenser electrostatically charges not only the coating particles, but the coating material in the hose, the coating material in the supply tank and the supply tank itself. Under these circumstances, operating personnel inadvertently coming into contact with an exposed supply tank or a charged hose or any other charged part of the system risk serious electrical shock unless such equipment is grounded to draw off the electricity. If the equipment is indeed grounded at any point, however, the electrostatics will not function because the high voltage charge would be conducted away from the coating dispenser electrode as well.
One of the methods for reducing the electrical shock problem is disclosed, for example, in U.S. Pat. No. 3,971,337 to Hastings which is by the same assignee as this invention. The Hastings patent discloses an apparatus for electrostatically isolating the supply tank which is connected to the coating dispenser. While this device is satisfactory for batch operations, it does not readily lend itself to continuous painting lines, i.e., applications wherein an essentially continuous supply of coating material must be provided over a period of time.
This problem has been addressed in apparatus of the type disclosed, for example, in U.S. Pat. No. 4,313,475 to Wiggins. In apparatus of this type, a "voltage block" system is employed wherein electrically conductive coating material is first transmitted from a primary coating supply into a transfer vessel which is electrically isolated from the spray gun. When filled with coating material, the transfer vessel is first disconnected from the primary coating supply and then connected to an inventory tank, which, in turn, is connected to one or more coating dispensers. The coating material is transmitted from the transfer vessel into the inventory tank to fill the inventory tank with a supply of coating material for subsequent transfer to the coating dispensers. While the inventory tank supplies the coating dispensers with coating material, the transfer vessel is disconnected from the inventory tank and connected back to the primary coating supply to receive another quantity of coating material so that the coating operation can proceed essentially continuously.
An important feature of apparatus of the type disclosed in the Wiggins U.S. Pat. No. 4,313,475 is that a voltage block or air gap is provided at all times between the primary source of coating material and the electrically charged coating dispensers. One potential operational problem with the Wiggins design is that separately actuated transfer devices, e.g., pneumatic cylinders or the like, are employed to interconnect the transfer vessel with the primary coating supply, and then to connect the transfer vessel with the inventory tank. Because the two pneumatic cylinders or other transfer devices are actuated independently of one another, it is possible that a malfunction of the controller for such cylinders could result in the connection of the transfer vessel to the primary coating supply at the same time the inventory tank is connected to the transfer vessel. As discussed above, the low resistivity of water-based coating materials can result in the transfer of a high voltage electrostatic charge from the coating guns, through a column of coating material to the primary coating supply, thus creating a hazard of electrical shock.
Another problem with apparatus such as disclosed in Wiggins U.S. Pat. No. 4,313,475 involves the leakage and/or drippage of coating material during the transfer process. As described above, the transfer vessel receives a supply of coating material from the primary coating supply, disengages the coating supply and then engages the inventory tank to transfer the coating material therein for supply to the coating dispensers. In the course of this transfer operation, the transfer vessel must make and break connections at both the primary coating supply and the inventory tank in order to effect the transfer of the coating material. It has been found that the connections and/or valving arrangements employed in such apparatus are susceptible to leakage and/or drippage, and thus present clean-up problems. In addition, leakage of such connections can result in grounding and thus loss of voltage in the electrostatic coating dispensers, and also could create an electrical shock hazard should a stream of dripping coating material contact an ungrounded object which can be touched by the operator.
Other potential operational problems with apparatus of the type disclosed in the Wiggins U.S. Pat. No. 4,313,475 involve handling of the coating material within the system. In such apparatus, the coating material is allowed to pool or come to rest within the transfer vessel and/or inventory tank. The pigments within coating material such as paints tend to settle if allowed to come to rest within a vessel or tank, and apparatus of the type disclosed in the Wiggins patent provide no means of circulating or moving the coating material within either the transfer vessel or inventory tank to maintain the pigments and other solids in suspension. Additionally, when the coating material such as paint is transferred between the vessels and tanks of the Wiggins apparatus, and to the coating dispensers, such movement is effected by the application of pressurized air within the vessel or tank to force the coating material therefrom. An air interface can degrade many types of paints and it is desirable to avoid contact with air until the coating material is applied to a particular substrate.